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Cap rock	A relatively impermeable (see permeability) rock, commonly shale, anhydrite or salt, that forms a barrier or seal above and around reservoir rock so that fluids cannot migrate beyond the reservoir. It is often found atop a salt dome.

casing	A large-diameter, relatively low-pressure pipe string set in shallow yet competent formations for several reasons. First, the surface casing protects fresh-water aquifers onshore. Second, the surface casing provides minimal pressure integrity, and thus enables a diverter or perhaps even a blowout preventer (BOP) to be attached to the top of the surface casing string after it is successfully cemented in place. Third, the surface casing provides structural strength so that the remaining casing strings may be suspended at the top and inside of the surface casing.

Casing	Relatively thin-walled, large diameter, steel pipe. Joints of casing are screwed together, run into a borehole, and cemented (see cementing) to the sides of the well. The casing is essential to withstand a variety of forces, such as collapse, burst, and tensile failure or chemically aggressive brines.

Cementing	Cement is pumped into place in a wellbore to permanently seal annular spaces (see annulus) between casing and borehole walls.

Coalbed methane	Coalbed methane (CBM), found in coal seams, as a source of unconventional natural gas (see unconventional resources).

Coiled Tubing	Metal pipes spooled on a large reel used to perform well interventions or also used as a production tubing in a depleted gas/oil well.

Collars	Collars are extremely thick-walled drill pipes having a higher weight than regular drill pipes. The drill collar section is the part right above the drill bit and downhole motor.

Collars

Completion	After a well is drilled it must be completed. This describes the process of placing a production tubing (see Production casing) in a well, perforating a well, stimulating (see stimulation) a well and removing all debris from the wellbore to allow a free flowing production stream. Completions can involve a variety of different services depending on the complexity of a well.

Condensates	Condensates are liquid hydrocarbon mixtures recovered from associated gas or non-associated gas. They are composed of C5 and higher carbon number hydrocarbons and normally have an API gravity greater than 45°.

Conventional resources	A ”normal“ reservoir, where the reservoir and fluid characteristics permit crude oil or natural gas to flow readily into wellbores. The term ”conventional” shall make a distinction from other unconventional resources such as shale or tight gas.

Corrosion	The loss of metal due to chemical or electrochemical reactions, which could eventually destroy a structure. Corrosion can occur anywhere in the production system, either at bottomhole or in surface lines and equipment. The corrosion rate will vary with time depending on the particular conditions of the oil field, such as the amount of water produced, secondary recovery operations and pressure variations. Special stainless steel qualities (i.e. non-magnetic steel) needed for Directional drilling are often confronted with pitting corrosion and stress corrosion.

Crude oil	A general term for unrefined petroleum or liquid petroleum. Crude oil is a hydrocarbon liquid with an API gravity of less than 45°. Condensates are hydrocarbon liquids with an API greater than 45°. Produced hydrocarbon liquids can generally be classified as Light oils with an API gravity greater than 31.1°. Intermediate oils with an API gravity from 31.1° to 22.3°. Heavy oils with an API gravity less than 22.3°. Crude oil with API gravity less than 10 °API is referred to as extra heavy oil or bitumen.

Cuttings	Rock flakes made by the drill bit.